In many industrial and commercial processes excess heat can be generated, and the heat can typically be removed from the process by means of cooling water. Comfort cooling of living and work spaces can generate excess heat that can be removed from the air conditioning equipment by means of cooling water. The term “cooling water” is thus utilized to describe water that flows through equipment to absorb and remove heat. Equipment can include, for example, air conditioning units, engine jackets, refrigeration systems, and industrial heat exchangers. Equipment can be found in, for example, in the glass, automotive, chemical, steel, and petroleum industries; as well as commercial properties.
Water, due to its low cost and physical properties, can be a suitable material for transfer of heat and use as an evaporative cooler. Unfortunately, warm water, with dissolved and suspended solids, can be a medium for growth of microorganisms. An uncontrolled growth of microorganisms in re-circulating cooling water systems can create several severe problems, for example, increased risk of Legionnaires' disease; plugging due to physical blockage of cooling water passages; accelerated corrosion under biological masses; and/or reduced heat exchanger efficiency due to bio-fouling of surfaces.
These problems can be amplified by an increased desire in various industries to minimize water usage and wastewater discharge via increasing the concentration (cycles) at which cooling towers are operated, and the use of reclaimed wastewater as cooling tower makeup water. The solids and nutrient content of the cooling water can increase when a cooling tower is operated at higher cycles and/or with reclaimed wastewater as makeup. This makes the cooling water environment even more conducive to microbiological growth.
Current microbial fouling control programs rely upon various oxidizing and non-oxidizing biocides, that while often effective, can have numerous problems, for example, high costs, severe health and safety concerns, low efficiency, and incompatibility with other chemical products needed to operate at higher cycles.
Oxidizing biocides, such as chlorine, ozone, and chlorine dioxide, while cost effective at low dosages, can have the following disadvantages or a combination thereof:
many oxidizers, such as chlorine, can be dangerous to handle;
most oxidizers can react with many of the common scale and corrosion inhibitors used in cooling water treatments;
organic oxidizers, such as hydantoin and n,n,dibromosulfamate, can be costly;
many oxidizers, such as ozone, can be volatile, resulting in higher usage and potential air pollution problems;
chlorine based oxidizers can have unwanted reactions with various organics, causing potential discharge problems.
In addition to these problems, chlorine based products can lose much of their effectiveness as the water pH increases. The increasing popularity of alkaline water treatment programs, commonly operated at pH levels about 8.0 su, can thus make chlorine based products unusable for biological control.
Non-oxidizing biocides, such as dithiocarbamate, isothiazolin, and glutaraldehyde; while avoiding some of the problems related to oxidizers, can have the following problems during application. Recent research has shown that non-oxidizers can be ineffective against the Legionnaires' disease bacterium. Non-oxidizers can be very high use cost products, some, such as isothiazolin, are very dangerous to handle. Many non-oxidizers can have very slow reaction times, making them impractical to use in short half-life systems. Due to development of resistant organism populations, non-oxidizers can lose effectiveness and may need to be rotated. Further, some non-oxidizers can be highly regulated due to potential environmental problems.
Previously, electrolytic cells have been constructed from graphite purchased from a number of suppliers, such as St. Marys Carbon Company and Carbone Lorraine, both of St. Marys, Pa. This graphite, of high purity but varying density, could be made into a plate, then impregnated with a resin, to make the plate impervious to the passage of water. It has been found that processes of impregnation have presented a number of problems. If the process involves placing the graphite plate in a vacuum-pressure chamber, drawing a vacuum for a period of 1 to 4 hours, introducing the impregnating resin, then pressurizing the chamber at up to 100 psig for a period of 1 to 24 hours, problems arise. These problems include problems with insufficient impregnation and insufficient penetration of the resin into the pores of the graphite plate. Upon release of pressure, treatment in an oven for 4 to 24 hours at temperatures up to 300° F., and repeating the entire process three times, problems were still encountered once the impregnated graphite plate was assembled into an electrolytic cell. Assembled cells using such impregnated plates would fail pressure tests with water at 80 psig for 24 hours, and the graphite plates would show leakage through the plates. Attempts to repair a leaking plate would require oven drying the plate to remove water, followed by a repeat of the entire impregnation process.
Furthermore, anisostatically pressed and extruded graphites exhibit both directional porosity and density gradations, and have been found to make inferior electrolytic cell components due to rapid overall breakdown and uneven wear, leading to premature failure of the cell.
Other problems with various electrolytic cell designs include problems with rectangular designs mounted horizontally. Such designs accumulate gas in the top of the cell producing a zone of little or no wear and causing a lack in passage of electrical current through that portion of the cell. The resultant increased amperage through the remaining surface area of the cell can accelerate wear of that surface and result in a quicker failure of the cell. Moreover, in addition to the low wear zone in the top of the cell from gas accumulation, a zone of very high wear has been found in the bottom of rectangular electrolytic cells. It is believed that this accelerated wear results from accumulation of elemental bromine, which is substantially heavier than water, at the bottom of an operating cell. Accelerated chemical attack on the graphite in the bottom of such cells results because elemental bromine is a very strong oxidizer.